Feed forward gauge control system for a rolling mill

ABSTRACT

A control method and apparatus for maintaining a uniform predetermined thickness of rolled sheet metal in a metal rolling mill where variations in sheet thickness are measured prior to its entry into the mill rolls. As a basic function, signals corresponding to the measured variations are fed forward to control pressure actuator means for the rolls, to provide corrections that eliminate the variations. The actuator means are also controlled by a secondary function that operates independently to slowly return the pressure value of the actuator means toward its original pre-set value as corrections are applied in response to measured variations. These two control functions are added together to form a composite change to the roll pressure actuactor means which controls the roll pressure and prevents it from exceeding its original pre-set value by an amount that would disturb the strip &#39;&#39;&#39;&#39;shape.&#39;&#39;&#39;&#39; The variations of the unrolled sheet thickness are measured accurately even though the pass angle seen by the entry gauge is constantly varying.

States Patent lligham et a1.

1 Mar. 11, 1975 Primary Examiner-Milton S. Mehr Attorney, Agent, orFirm-Owen, Wickersham & Erickson [57] ABSTRACT A control method andapparatus for maintaining a uniform predetermined thickness of rolledsheet metal in a metal rolling mill where variations in sheet thicknessare measured prior to its entry into the mill rolls. As a basicfunction, signals corresponding to the measured variations are fedforward to control pressure actuator means for the rolls, to providecorrections that eliminate the variations. The actuator means are alsocontrolled by a secondary function that operates independently to slowlyreturn the pressure value of the actuator means toward its originalpre-set value as corrections are applied in response to measuredvariations. These two control functions are added together to form acomposite change to the roll pressure actuactor means which controls theroll pressure and prevents it from exceeding its original pre-set valueby an amount that would disturb the strip shape." The variations of theunrolled sheet thickness are measured accurately even though the passangle seen by the entry gauge is constantly varying.

12 Claims, 3 Drawing Figures CONVERTER PAYOF F TENSiO l l I I I I MAINDRIVE MOTOR PAYOFF MOTOR SPEED MEASUREMENT REWIND I MOTOR I DIRECTION OFSTRIP TRAVEL PAIEIITEIII I I 11% 3 869 as 2 sum 1 o 2 OPERATOR OPERATORSDESIRED STATION PRESSURE E 8 40 2 Q J 3 FEED FORWARD CONTROL SECTION 605 I 62 64 66 68 7O MILL z SPEED PAss- ROLL GAP AUTO R LINE TARGET LIMITRAMP OUTPUT ENTRY DELAY ANGLE CHANGE '"CHKG. RETURN FUNQ FIUNC.CORREC'I'. coMP. FUNC 12 I FEEDBACK CONTROL I 46 I I 44 I L ROLL GAPDIGITAL To ANALOG To CONTROLLER c a f 2 g; R I DIGITAL CONVERTER z 38 QI I 2 30 2 2o 1; n. I 2 22 '2 34 2s 28 REWIND MAIN DRIVE PAYOFF MOTORSPEED MOTOR MOTOR MEASUREMENT J FIG.. 1

DIRECTION OF STRIP TRAVEL PAI'ENTEDHARI 1 W5 3,869,892

sum 2 g g THICKNESS I I IE 22 I I2 I Y (a) MEAS. AT ENTRY GAUGE 74THICKNESS AT ROLL ORT I DELAY BITE IF NO CORR. \76

FROM FEED FWD.

78 AUTOMATlC RAMP OUTPUT or FEED FW RETURN (c) TO ROLL GAP CONTROLLER II PRESS. TIME CONT.

(d) ROLL PRESSURE A 79 I THICKNESS LEAVING STEADY STATE ROLL am: I ERRoRDUE TO RAMPI I l T a a n l THICKNESS MEAS. I I \I AT EXIT GAUGE FIG 2 Iso (RADIATION SOURCE) |5-u-I I-52(DETECTORI 3 FEED FORWARD GAUGE CONTROLSYSTEM FOR A ROLLING MILL In the rolling of sheet-metal to apredetermined thickness the major reduction in thickness is accomplishedby the mill rolls which are forced together by some controllablepressure means. Variations in thickness of the sheet metal stock to berolled must be accounted for during the rolling process in order toobtain a rolled end product of uniform thickness. Measurement of suchvariations prior to entry of sheet material into the pressure rolls isessential so that feed forward corrections can be made.

One problem heretofore encountered by certain rolling mill controlsystems related to maintaining the proper cross-sectional shape andthickness of the sheet material. In such mills the rolls are tapered sothat the proper cross-sectional shape of material is maintained onlywhen the pressure of the rolls is held within certain limits. Wherethickness variations in the supply material required increased rollpressure to control the output sheet thickness, a distortion of therolls sometimes occurred where the pressure required was excessive. Thiscaused the rolls to bend, thereby resulting in a cross-sectionalvariation in sheet thickness. Yet, if the pressure of the rolls wasexcessively diminished, a distortion in the cross-sectional shape in theopposite direction occurred. The present invention solved this problem.

Another problem with prior feed forward control systems was that theydid not provide any correction for a varying passline angle through theinput gauge. In most rolling mills the material removed from supplyrolls passes through a varying passline angle with the horizontal beforeit enters the rolls and does not pass through either a constant angle ora horizontal path for any appreciable distance before entering thepressure rolls. Thus, in such mills it is impossible or impractical tolocate a gauge where the material passline is not varying as the supplyroll diameter constantly decreases. The present invention eliminatesthis problem so that the gauge reading used for the feed forward controlis essentially not affected by a varying passline angle.

OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, a general objectof the present invention to provide an improved method for controllingthe gauge of metal material being rolled to a predetermined thicknessfrom rolled stock of a greater thickness and having random thicknessvariations.

Another more specific object of the invention is to provide a method asabove which measures the sheet thickness of material prior to its entrybetween pressure rolls even where a varying passline is seen by theentry gauge.

Yet another object of the present invention is to provide a method forcontrolling the uniform thickness of metal being rolled in a rollingmill wherein the measured entry thickness is utilized to control thepressure of the rolls so as to provide the desired material thick nessdespite thickness variations, and more specifically wherein when suddenvariations in incoming thickness occur, the roll pressure is abruptlyincreased when the variation reaches the roll bite to rapidly eliminatethe variation in thickness and immediately thereafter the pressure isautomatically returned at a relatively slow rate toward the operator'sset point, thereby generating a small error in thickness at the outputgauge of the mill to cause the feedback controller to compensate bychanging payoff tension and/or mill speed.

The apparatus for accomplishing the aforesaid method steps generallycomprises a fixed noncontacting radiation gauge of the X-ray or nuclearisotope kind that is mounted near the payoff reel. The strip materialbeing rolled has an angular, varying passline through the entry gaugewhich generates measurement signals as a result of absorption of nuclearparticles in the strip. The measurement signals are generatedintermittently at a constant rate (e.g. 20 times per second), and areconverted into a measure of the variations in strip thickness.

These signals from the entry gauge are supplied to a high speed feedforward section of a computer system having a series of function blockswith inputs controllable from an operators station having videomonitoring facilities. The computer system provides output controlsignals to a pressure controller which is connected to a pressureactuating system on the mill rolls and it also provides signals to thespeed and tension control means on the mills.

The angle of the strip passing through the X-ray gauge at entry affectsboth the absolute thickness seen by the gauge and also the differencebetween one reading and the next. The differential of the gauge readingis calculated assuming nominal entry gauge thickness and this value isall that is required for feedforward. The differential entry gaugereading is delayed by a time delay, representing the transport delay ofthe mill less the time constant of the pressure controller. Delayedchanges in entry gauge are used to modify the set point of the pressurecontroller. The transport delay is modified with mill speed. The gain ismodified by strip width and thickness, and also by the alloy beingrolled.

Further objects, advantages and features of the present invention willbecome apparent from the following detailed description taken inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic and block diagramof a feed forward control system embodying the principles of the presentinvention;

FIG. 2 is a diagram of my feed forward system showing the action ofvarious parameters during typical operation;

FIG. 3 is a diagrammatic view of the entry gauge for providingmeasurement signals of strip material with a varying angular passline.

DETAILED DESCRIPTION OF EMBODIMENT General With reference to thedrawing, FIG. 1 shows in block diagram form a control system for a metalcold-rolling mill 10 which in broad terms comprises a payoff reel 12 ofthe metal strip material 14, such as aluminum or copper which is to berolled to a smaller uniform thickness. The strip is threaded through anentry guage l6 and thence between the bite of a pair of work rolls l8and 20 and an exit gauge 22 to a rewind reel 24. The latter has asuitable power drive 26 and means to increase tension of the stripbetween the rewind reel and the work rolls. The payoff reel 12 has asimilar drive 28 including means to increase tension between it and thework rolls. The strip engaging work rolls 18 and 20 are pressed togetherby a pair of back-up rolls 30 and 32 in the conventional manner. Theback-up rolls are supported in chocks (not shown) and they aremechanically controllable to vary the gap between the work rolls. One ofthe work rolls 20 is mechanically connected to a main drive motor 34having a connected speed measurement device 36 that provides an outputequivalent to actual mill speed. A system is provided for moving onework roll toward the other and against the metal strip 14 with theparticular pressure necessary to maintain the preselected roller gap.This system, though not shown in detail, is well known to those skilledin the metal rolling art and typically includes a hydraulically poweredactuator (not shown) with a preload cylinder 38.

The mill is connected to a feed forward control section 40 of a computervia a computer/mill interface including an analog to digital converter42 connected to the entry gauge 16. This interface also includes a rollgap controller 44 that controls the actual pressure which the rollsapply to the strip material and hence the thickness of the metal comingout of the roll bite. This controller receives input control signalsfrom a computer system through a digital to analog converter 46 which ispart of the computer/mill interface.

As shown in FIG. 1, the computer feed forward control section 40receives input data from the entry gauge 16 located near the payoff reel12. This section comprises a series of calculating or functionalelements of the computer which process data received from thecomputer/mill interface and an operators station 48. These functionalcomputer elements compute a new target change for the roll gapcontroller 44 via the computer/mill interface.

A more'detailed description of the elements at the operators station 48as well as those comprising the feedforward control section 40 will nowbe presented.

The Entry Gauge 16 An important aspect of the present feed forwardsystem is that thickness measurements of the strip metal 14 are made asit leaves the payoff reel 12 and are fed forward through the computersystem 40 to control small, short term variations of pressure on themill work rolls l8 and by the roll group control 44. The thicknessmeasurements between the payoff reel 12 and the work rolls are made bythe gauge 16 which may, of necessity, be located closely adjacent to thepayoff reel where the metal strip being measured passes through thegauge at a varying passline angle. The signal from the entry gauge isread at a high frequency synchronized to the execution rate of thefunctions in the section 40 of FIG. 1, which is typically 20 times persecond.

The gauge itself is preferably of the non-contacting radiation typeutilizing either an X-ray or nuclear isotope radiation source. One formof such a gauge is shown in US. Pat. No. 3,757,122.

As shown in FIG. 3, a correction to the thickness measurement for thepass angle of metal through the entry gauge 16 is accomplished bycombining the actual measurement of the gauge with an assumption thatthe actual thickness of the strip is near to target thickness, in thefollowing manner:

For a radiation measuring device such as shown in FIG. 3, the equationrelating thickness of material between a radiation source 50 and adetector 52 to the radiation received by the detector is generally knownto those familiar with the art as the gauge equation and is expressed asfollows:

where T= measured thickness I R measured ratio between the voltageprovided by the detector with material present, and the voltage withonly air present, subsequently referred to as the ratio.

,u. is a constant depending on the absorption characteristics of thematerial being measured.

Referring to FIG. 3, which shows material at an angle other than to thesource detector time:

T thickness of material which the radiation must penetrate.

T actual thickness of material. By considering a small change in actualthickness A T,,, which results in a small change A T in thickness forthe radiation to penetrate, the gauge equation (l) can be differentiatedto produce the relationship.

A TM /I M) A RM Where R is the measured ratio for thickness T and A R isa small change in that measured ratio. From basic geometry (FIG. 3)

TA/TM A TA/A TM Combining equations (2) and (3) by basic algebra A TATA/TM /I M) A RM But, from the basic gauge equation (I),

TA/TM low A/loge RM Now, combining equations (4) and (5):

A T l/uR (log R /log R A R Where R is the ratio R as previously definedif the metal is presented to the gauge perpendicular to the radiationbeam; and

R is the ratio R actually measured with the strip at some angle to theradiation beam.

This equation relates a change in actual thickness of the material to achange in measured ratio taken at an angle other than 90 to the gauge.

It it is assumed that R,, is approximately equal to the ratiocorresponding to the nominal thickness of metal entering the mill, whichmay be entered by the operator, then the magnitude of thickness changesof the metal strip entering the mill can be obtained by measuring theratio R Thus, in the method of the present invention, the input gaugemeasures the actual thickness (T to obtain the ratio R even though theangle of the strip through the gauge (the passline angle) may Theoperators station48 normally located within an enclosure near the millincludes suitable apparatus comprising:

a. means for selecting feedforward control on or off;

b. means for entering the nominal entry thickness of the metal beingrolled into the mill into the computer system;

c. means for entering the roll pressure target, which the operatordesires for good strip shape, into the computer system; and

d. a means for displaying all selected inputs for checking purposes.

These functions are preferably implemented with a video display andkeyboard entry station. Since all of the aforesaid apparatus andarrangement of same is well known to those skilled in the art, it is notshown in detail.

The Computer Feed Forward Control Section 40 The feed forward controlsection 40 of the computer system comprises: a. Transport Delay Function60.

The instant speed of the mill, measured via the speed measurement device36 and furnished to the analog to digital converter 42 is supplied tothe transport delay function block 60.

Using this speed, the block 60 computes the transport time of the stripfrom the measurement point to the roll bite. This may be expressed bythe following equation:

where d is the distance from the measurement point 16 to the roll bite18 and v is the strip velocity. In order to make a compensatoryadjustment to the roll gap to eliminate the gauge disturbance it takes afinite time for the roll gap controller 38 to move. This time isreferred to as the roll gap time constant t To allow for this, the timedelay r calculated by block 60 is reduced by this amount and usingequation (7) the computation in block 60 is r (d/v) r The voltagereceived from the entry gauge for the initial correction is delayed bythe amount t computed by this transport delay function by setting up ashift register in the computer software in the conventional manner. Thisshift register is shifted at a fixed execution frequency which is thesame as that of the other functions of section 40 in FIG. I, typically20 times per second. Various values of the 1. are represented by takinga value out of the shift register at a point down the registerproportional to r For example, if 2,, 735 milliseconds and the executiontime is once every 50 milliseconds the time delay is represented by avalue N items down the shift register where The fractional part of N ishandled by linear interpolation of the 14th and 15th values in the shiftregister in the conventional manner. b. Pass Angle Correction Function62.

The nominal entry thickness is entered into a pass angle function block62 of the computer by the operator at the operators station 48. Here,this input data is used in conjunction with calibration constant heldwithin the computer to convert the delayed change in entry gaugemeasurement from the block 62 into a thickness change as described inequation (6) in the previous section on the entry gauge.

For example, ifA R 0.01, R,, 0.6, R 0.5 and p. 0.4 then A TA 1/0.4 0.5lo 0.6/10 0.5 0.01

0.0368 which is the amount of thickness change that will be entering theroll gap, the units being thousandths of an inch if the units of ,u. are(thousandths of an inch) c. Roll Gap Target Change Computation 64.

From the thickness change input, provided by the pass angle functionblock 62, a change in roll gap is computed in order to eliminate thisthickness change. This is a straightforward computation using previouslydetermined values and calculations relating thickness out of the mill toroll gap target, strip width, strip hardness, and strip thickness.

d. Limit Checking 66.

For operational safety of the mill and to maintain strip quality, it isimportant that the roll pressure does not depart too far from the valueP originally set in by the operator. An excessive change in pressurewill cause work roll bending and will alter the shape of the strip. Onthe other hand the feedforward control section needs to make somepressure change in order to effect a gauge connection. Block 66therefore checks the absolute value of the difference between thedemanded roll gap pressure P and the operators value P and prevents thisvalue from exceeding a preset limit P by restricting the value of P.

Expressed as equations this process is as follows: At

control period n, block 64 generates a required-change- P ix-1 A Pu Thevalue of P" is then limit checked as follows:

9f 1 P'" P l P then P" P'" then P" P P if P" P The value of P isdifferent for different metal thicknesses and' alloys and so thecomputer selects the ap propriate value from a table based on the actualallo and thickness being rolled. 6. Automatic Ramp Return Function 68.

Up to this point the feedforward control function has calculated achange to the roll gap controller target to completely eliminate changesin entry gauge thickness as the metal is rolled. Now. an automatic rampreturn function block 68 compares the roll pressure P desired by theoperator (for reasons of good strip shape) with the computed roll gappressure P" and superimposes upon this computed pressure a change whichcauses the roll pressure to slowly return or ramp back to the operatorsdesired roll pressure.

This would result in a slowly increasing error in exit gauge of thestrip since we are slowly removing the corrective pressure changerequested by the feedforward control section 40. However, this gaugeerror is seen by a standard feedback controller 72 conventionallypresent in rolling mills, which corrects for the'error by adjustingother mill variables such as speed or tension. The effect of the autoramp return function 68 is to slowly transfer any long term changes inroll pressure required by changing incoming strip thickness to othervariables that also control thickness such as speed or tension. Becausethe feedback controller 72 does not have an instantaneous response, theeffect of the slow ramp return 78 of pressure by block 68, is to cause asmall steady state gauge error 79 during the ramp return (See FIG. 2).The ramp is slow enough that this error 79 is negligibly small. Tocompletely eliminate this error it would be possible to send a signaldirectly from block 68 to the payoff tension or speed used by thefeedback controller 72. However, in practice, the size of this gaugeerror does not require this additional complexity. 1 f. Output Function70 The output function takes the resultant composite change in roll gaptarget and outputs it through the digital to analog converter 40 to theroll gap controller 44.

The computation described and embodied by the elements of section 40 ofFIG. 1 are typically made 20 times per second. This frequency may varyand is chosen so that a reasonable number of executions are made in thetime taken for the strip to travel from the entry gauge to the rollbite.

The aforesaid thickness error caused by the ramping back action ismeasured by the exit gauge 22 which is connected to the feedbackcontroller 72. This controller receives an input from the analog todigital converter 42 equivalent to actual mill speed, and it suppliesfeedback outputs to the main drive motor 34 and to the pay-off drivemotor 28 which also controls tension of the strip material. Thus,control action taken by the feedback controller 72 substantially removesthe thickness error introduced by block 68 described above by changingother mill control values which affect thickness (e.g., pay-off, tensionand mill speed control).

The overall combined effect of the feedforward and feedback controlsystems is illustrated graphically in FIG. 2.

Assume that a sudden change in incoming thickness occurs at the entrygauge 16, as indicated at 74 on the upper trace (a) of the diagram. Whenthis change reaches the roll bite the pressure of the work rolls 18 and20 is increased abruptly as indicated at 76 of next lower trace (b) toeliminate the change in thickness. Simultaneously, the automatic rampreturn function is automatically set up to commence the steady return ofroll pressure toward the operators setpoint, as indicated by the ramp 78of the trace (c). The actual roll pressure change is shown in trace (d)of FIG. 2. This ramping along line 78 generates a small error inthickness out of the mill which causes the feedback controller 72 tocompensate in its normal manner by changing the pay-off tension at thecontrol 28 and/or mill speed by the main drive motor 34. Thus, inaccordance with the present invention, the initial control action istaken by first changing the roll pressure which is capable of changingat a fast rate. Then, this initial change of roll pressure is slowlytransferred to the pay-off tension and mill speed. The resultingvariation in strip thickness are shown at the roll bite and at the exitgauge in traces (c) and (f) of FIG. 2. The unique combination of feedforward and feedback control serves to keep the roll pressure of a millwithin limits consistent with good strip shape while maintaining a highdegree of accuracy on the rolled output thickness of the metal stripmaterial.

To those skilled in the art to which this invention re lates, manychanges in construction and widely differing embodiments andapplications of the invention will suggest themselves without departingfrom the spirit and scope of the invention. The disclosures and thedescription herein are purely illustrative and are not intended to be inany sense limiting.

We claim:

1. A method for controlling the gauge without changing the shape ofstrip material issuing from a rolling mill that includes a payoff reel,a pair of pressure rolls to produce a desired roll force, pressurecontrol means for varying the pressure of said rolls on the stripmaterial and the gap between them, a takeup reel, an input gauge betweensaid payoff reel and said rolls, tension control means for the stripmaterial, an output gauge and feedback means connected to said outputgauge, said method comprising the steps of:

generating a signal at the input gauge for determining a variation inthe gauge of material from the payoff reel from a nominal thickness;producing a primary control function causing said pressure control meansto operate in response to the variation in gauge detected by said inputgauge when the actual gauge variation reaches said pressure rolls, saidprimary control function operating on said pressure rolls to changetheir force on the strip at a relatively rapid rate to substantiallyeliminate the thickness variation;

superimposing a secondary function on said pressure control meansimmediately after the activation of said primary function, saidsecondary function causing the changed force of the pressure rolls'onthe strip material by said primary control function to commencereturning at a relatively slow steady rate toward the original presetlevel and thereby causing a relatively small variation in target gaugeof the material passing from said rolls;

generating an error signal proportional to said small variation at theexit gauge; and

utilizing said error signal to activate a mill control means foreliminating said small variation in gauge.

2. The method as described in claim 1 wherein said error signal isutilized to operate the tension control means to counteract said smallvariation in gauge.

3. The method as described in claim 1 wherein said error signal isutilized to vary the speed of strip material in the mill to counteractsaid small variation in gauge.

4. The method as described in claim 1 wherein the signal is generated atthe input gauge by measuring the actual thickness of the strip as itpasses at a varying angle between the radiation source and detectorelements of the entry gauge and using said signal from the input gaugein conjunction with the assumed nominal thickness value of the stripmaterial to compute the actual variation in incoming strip thickness.

5. The method as described in claim 4 wherein said actual variation inincoming strip thickness is determined by the equation T4 (log R /klg RA R where: u a constant depending on the absorption characteristics ofthe material being measured; R is the ratio between the voltage providedby the detector with material present and the voltage with only airpresent when the metal is perpendicular to the detectors radiation beam;R is the ratio between the voltage provided by the detector withmaterial present and the voltage with only air present which is actuallymeasured with the strip at an angle to the radiation beam; A R is thecurrent R minus the previous R 6. In a rolling mill including a payoffreel for a strip of sheet material connected to a takeup reel, a pair ofpressure rolls for applying a forming force on said strip material and apressure control means for controlling said pressure rolls, an entrygauge between said payoff reel and said rolls and an exit gauge betweensaid rolls and said takeup reel, a feed forward thickness control systemfor said mill comprising:

means for utilizing signals from said entry gauge and comparing themwith an input proportional to the assumed nominal thickness of the stripmaterial from said payoff reel to produce signals proportional tovariations in thickness of the strip material; primary control means forutilizing said signals proportional to said strip thickness variationsfor operating said pressure rolls control means to cause a relativelyrapid increase in pressure by said rolls;

means responsive to activation of said pressure control means forcommencing a steady reduction of roll pressure; feedback control meansresponsive to signals generated by said exit gauge compared with apreselected target gauge for producing an error signal;

and means responsive to said error signals for activating another gaugecontrol means on the mill. 7. The method as described in claim 6 whereinsaid primary control function is produced by:

computing the transport time of a gauge variation from the measurementpoint to the roll bite and subtracting from this time a roll gap timeconstant;

providing a pass angle correction to the nominal entry thickness andusing it in conjunction with a calibration constant to convert thedelayed change in entry gauge measurement into a thickness change;

computing a change in the roll gap based on the previously computedthickness change modified by characteristics of the strip material; and

checking the computed roll pressure for compliance with operationallimits.

8. An apparatus for controlling the gauge and shape of strip materialissuing from a rolling mill that includes a payoff reel, a pair ofpressure rolls adjustable to a preselected gap to produce a desiredtarget thickness, pressure control means for varying the pressure ofsaid rolls on the strip material and the gap between them, a takeupreel, an input gauge between said payoff reel and said rolls, tensioncontrol means for the strip material, an output gauge and feedback meansconnected to said output gauge, said apparatus comprising:

means for generating a signal at the input gauge for determining avariation in the gauge of material from the payoff reel from a nominalthickness;

means responsive to said signal from said input gauge for producing aprimary control function causing said pressure control means to operatewhen the actual gauge variation reaches said pressure rolls to changetheir force on the strip at a relatively rapid rate to substantiallyeliminate the thickness variation; means for superimposing a secondaryfunction on said pressure control means immediately after the activationof said primary function, said secondary function causing the changedforce of the pressure rolls on the strip material by said primarycontrol function to commence returning at a relatively slow steady ratetoward the original preset level and thereby causing a relatively smallvariation in target gauge of the material passing from said rolls;

means for generating an error signal proportional to said smallvariation at the exit gauge; and

means for utilizing said error signal to activate a mill control meansfor eliminating said small variation in gauge.

9. The apparatus as described in claim 8 including means for utilizingsaid error signal to operate the tension control means to counteractsaid small variation in gauge.

10. The apparatus as described in claim 8 including means for utilizingsaid error signal to vary the speed of strip material in the mill tocounteract said small variation in gauge.

11. The apparatus as described in claim 8 wherein said means forgenerating a signal at the input gauge includes means for measuring theactual thickness of the strip as it passes at a varying angle betweenthe radiation source and detector elements of the entry gauge and meansfor using said signal from the input gauge in conjunction with theassumed nominal thickness value of the strip material to compute theactual variation in incoming strip thickness.

12. The apparatus as described in claim 8 wherein said means forsuperimposing a secondary function comprises an automatic ramp returnmeans for generating a signal which moves the pressure control means ata preselected constant rate towards an original preset level until thatpreset level has been reached.

1. A method for controlling the gauge without changing the shape ofstrip material issuing from a rolling mill that includes a payoff reel,a pair of pressure rolls to produce a desired roll force, pressurecontrol means for varying the pressure of said rolls on the stripmaterial and the gap between them, a takeup reel, an input gauge betweensaid payoff reel and said rolls, tension control means for the stripmaterial, an output gauge and feedback means connected to said outputgauge, said method comprising the steps of: generating a signal at theinput gauge for determining a variation in the gauge of material fromthe payoff reel from a nominal thickness; producing a primary controlfunction causing said pressure control means to operate in response tothe variation in gauge detected by said input gauge when the actualgauge variation reaches said pressure rolls, said primary controlfunction operating on said pressure rolls to change their force on thestrip at a relatively rapid rate to substantially eliminate thethickness variation; superimposing a secondary function on said pressurecontrol means immediately after the activation of said primary function,said secondary function causing the changed force of the pressure rollson the strip material by said primary control function to commencereturning at a relatively slow steady rate toward the original presetlevel and thereby causing a relatively small variation in target gaugeof the material passing from said rolls; generating an error signalproportional to said small variation at the exit gauge; and utilizingsaid error signal to activate a mill control means for eliminating saidsmall variation in gauge.
 1. A method for controlling the gauge withoutchanging the shape of strip material issuing from a rolling mill thatincludes a payoff reel, a pair of pressure rolls to produce a desiredroll force, pressure control means for varying the pressure of saidrolls on the strip material and the gap between them, a takeup reel, aninput gauge between said payoff reel and said rolls, tension controlmeans for the strip material, an output gauge and feedback meansconnected to said output gauge, said method comprising the steps of:generating a signal at the input gauge for determining a variation inthe gauge of material from the payoff reel from a nominal thickness;producing a primary control function causing said pressure control meansto operate in response to the variation in gauge detected by said inputgauge when the actual gauge variation reaches said pressure rolls, saidprimary control function operating on said pressure rolls to changetheir force on the strip at a relatively rapid rate to substantiallyeliminate the thickness variation; superimposing a secondary function onsaid pressure control means immediately after the activation of saidprimary function, said secondary function causing the changed force ofthe pressure rolls on the strip material by said primary controlfunction to commence returning at a relatively slow steady rate towardthe original preset level and thereby causing a relatively smallvariation in target gauge of the material passing from said rolls;generating an error signal proportional to said small variation at theexit gauge; and utilizing said error signal to activate a mill controlmeans for eliminating said small variation in gauge.
 2. The method asdescribed in claim 1 wherein said error signal is utilized to operatethe tension control means to counteract said small variation in gauge.3. The method as described in claim 1 wherein said error signal isutilized to vary the speed of strip material in the mill to counteractsaid small variation in gauge.
 4. The method as described in claim 1wherein the signal is generated at the input gauge by measuring theactual thickness of the strip as it passes at a varying angle betweenthe radiation source and detector elements of the entry gauge and usingsaid signal from the input gauge in conjunction with the assumed nominalthickness value of the strip material to compute the actual variation inincoming strip thickness.
 5. The method as described in claim 4 whereinsaid actual variation in incoming strip thickness is determined by theequation TA - 1/ Mu RM (loge RA/loge RM) Delta RM where: Mu a constantdepending on the absorption characteristics of the material beingmeasured; RA is the ratio between the voltage provided by the detectorwith material present and the voltage with only air present when themetal is perpendicular to the detector''s radiation beam; RM is theratio between the voltage provided by the detector with material presentand the voltage with only air present which is actually measured withthe strip at an angle to the radiation beam; Delta RM is the current RMminus the previous RM.
 6. In a rolling mill including a payoff reel fora strip of sheet material connected to a takeup reel, a pair of pressurerolls for applying a forming force on said strip material and a pressurecontrol means for controlling said pressure rolls, an entry gaugebetween said payoff reel and said rolls and an exit gauge between saidrolls and said takeup reel, a feed forward thickness control system forsaid mill comprising: means for utilizing signals from said entry gaugeand comparing them with an input proportional to the assumed nominalthickness of the strip material from said payoff reel to produce signalsproportional to variations in thickness of the strip material; primarycontrol means for utilizing said signals proportional to said stripthickness variations for operating said pressure rolls control means tocause a relatively rapid increase in pressure by said rolls; meansresponsive to activation of said pressure control means for commencing asteady reduction of roll pressure; feedback control means responsive tosignals generated by said exit gauge compared with a preselected targetgauge for producing an error signal; and means responsive to said errorsignals for activating another gauge control means on the mill.
 7. Themethod as described in claim 6 wherein said primary control function isproduced by: computing the transport time of a gauge variation from themeasurement point to the roll bite and subtracting from this time a rollgap time constant; providing a pass angle correction to the nominalentry thickness and using it in conjunction with a calibration constantto convert the delayed change in entry gauge measurement into athickness change; computing a change in the roll gap based on thepreviously computed thickness change modified by characteristics of thestrip material; and checking the computed roll pressure for compliancewith operational limits.
 8. An apparatus for controlling the gauge andshape of strip material issuing from a rolling mill that includes apayoff reel, a pair of pressure rolls adjustable to a preselected gap toproduce a desired target thickness, pressure control means for varyingthe pressure of said rolls on the strip material and the gap betweenthem, a takeup reel, an input gauge between said payoff reel and saidrolls, tension control means for the strip material, an output gauge andfeedback means connected to said output gauge, said apparatuscomprising: means for generating a signal at the input gauge fordetermining a variation in the gauge of material from the payoff reelfrom a nominal thickness; means responsive to said signal from saidinput gauge for producing a primary control function causing saidpressure control means to operate when the actual gauge variationreaches said pressure rolls to change their force on the strip at arelatively rapid rate to substantially eliminate the thicknessvariation; means for superimposing a secondary function on said pressurecontrol means immediately after the activation of said primary function,said secondary function causing the changed force of the pressure rollson the strip material by said primary control function to commencereturning at a relatively slow steady rate toward the original presetlevel and thereby causing a relatively small variation in target gaugeoF the material passing from said rolls; means for generating an errorsignal proportional to said small variation at the exit gauge; and meansfor utilizing said error signal to activate a mill control means foreliminating said small variation in gauge.
 9. The apparatus as describedin claim 8 including means for utilizing said error signal to operatethe tension control means to counteract said small variation in gauge.10. The apparatus as described in claim 8 including means for utilizingsaid error signal to vary the speed of strip material in the mill tocounteract said small variation in gauge.
 11. The apparatus as describedin claim 8 wherein said means for generating a signal at the input gaugeincludes means for measuring the actual thickness of the strip as itpasses at a varying angle between the radiation source and detectorelements of the entry gauge and means for using said signal from theinput gauge in conjunction with the assumed nominal thickness value ofthe strip material to compute the actual variation in incoming stripthickness.